Fossil fuel sources (coal, oil, natural gas, LPG, and other hydrocarbons) are burned to generate the majority of electrical power in the world. Burning fossil fuels releases a number of pollutants into the atmosphere that are hazardous:                carbon dioxide is a greenhouse gas and contributes to global warming;        nitrogen oxides (NOx) contribute to smog; and        sulfur oxides (SOx) contribute to acid rain.        
In addition, the supply of such fuels is finite and the cost of extracting incrementally greater amounts from the earth is accompanied with increasing cost.
Nuclear fission requires fissionable material, the supply of which is also finite. In addition nuclear power generates radioactive waste which is costly to store since it must be located away from human populations, hermetically sealed, and monitored for 100,000s of years.
Hydroelectric power is renewable and has zero emissions; however, moving bodies of water must be dammed to harness this power source. There are a finite number of viable moving bodies of water in the world, most of which are already dammed. In addition, dams hinder the spawning of certain fish species.
Photovoltaics can be used to generate electrical power directly from incident solar radiation; however, it is expensive to manufacture and install large arrays of photovoltaics. The current cost/performance metrics for photovoltaics are not sufficient for large-scale implementation.
Solar-thermal methods require high temperatures to realize high Carnot efficiency. Achieving such high temperatures requires expensive actively controlled light focusing systems.
Other, zero-emission, renewable energy sources, such as wind, wave, and geothermal exist, but are limited to certain geographical regions. Wind power requires high winds, wave power requires substantial waves, tidal power requires large height changes between high and low tide, and geothermal requires a region of high geothermal activity. None of these sources is abundant enough to have the potential to meet the entire current world energy demand, let alone the future world energy demand.
Ground source heat can be a plentiful source of energy, but capital cost of equipment can be high as can be the cost of drilling large bore holes deeply where high-grade heat is located. Less expensive near-surface heat sources provide low grade heat which is not efficiently converted to electrical power.
In addition, some power generation methods require significant amounts of fresh water in the power-generation process.
Schemes have been proposed to use capillary action to elevate water and thus add potential energy to water. These schemes usually propose to use this water to turn a turbine and generate power as the water is decreased in elevation. Such schemes are thermodynamically impossible. (They do not account for the energy required to separate the fluid from the capillary pores once its elevation has been increased.) See page 555 of vol. 18, 9th ed. the Encyclopedia Britannica by Thomas Spencer (1888).